1. Field of the Invention
This invention relates to a method of improving the quality and yields of products from a delayed coking process.
2. Description of the Prior Art
The fundamental reactions involved in a delayed coking process are thermal cracking and polymerization of petroleum residues under conditions of pressure, temperature, and time. The process produces green coke, hydrocarbons and gases--the quality and quantity of which are mainly dependent upon the type and quality of the fresh feed charged into the unit. During operation, this fresh feed is rapidly heated in a coker heater above its cracking temperature, or approximately 920.degree. F., within 5 minutes. Its effluent is then transferred into a large soaking vessel called a coke drum which allows the reactions to proceed for approximately 24 hours.
For obvious economic reasons, it has been the objective of a refinery, and the subject of many inventions, to increase the yield of hydrocarbons and to lower the yield of less valuable green coke. The gas yield meanwhile attains a narrow and economically insignificant range which is virtually unchanged over a given set of operating conditions in the coke drum and therefore, is given less attention in the unit. Hydrocarbons yield enhancement is specially emphasized for stocks producing fuel grade and aluminum anode grade cokes. For premium or needle cokes, the emphasis is mostly on improved end-user coke properties such as lower coefficient of thermal expansion (CTE), higher mechanical strenght and higher electrical conductivity. For any given coker fresh feed, and any given set of operating conditions, a refiner has limited control of improving green coke quality while optimizing product yields. Among many of the most recent inventions in yield optimization are U.S. Pat. Nos. 4,455,219, 4,518,487, 4,549,934, and 4,604,186. The former first three patents emphasize partial or complete removal of the natural heavy recycle oil from the coker heater charge while the latter fourth patent teaches the blending of hydrogen donor hydrocarbons to the coker fresh feed.
The availability of heavy and low quality coker fresh feeds have given rise to the production of fuel grade green cokes which are spherical in shape. Its size may vary from a sand particle to buck shots, and invariably, up to a size as large as a basketball. It is called "shot coke" in the industry and considered undesirable, though normally tolerated, during operation. Shot coke formation tends to produce less hydrocarbons and more green coke. A more detailed discussion on the commercial significance of shot coke and its ramifications is reported in the Oil & Gas Journal, July 27, 1987, pages 51-56, "Delayed coking is topic of experience exchange".
Constituents of heavy petroleum residues typically consist of asphaltenes, resins and heavy oil. The latter is medium aromatics of approximately 2 to 4 benzene rings, 0 to 2 naphthene rings and cycloparaffins, and some attached alkyl substituents. Resins has more benzene rings and alkyl substituents than heavy oil, but asphaltenes has the most benzene rings, understably the heaviest, and the constituent where most of the green coke yield is derived. Clusters of aromatic rings of various sizes in any of these constituents may be connected by aryl and alkyl bridges which separates when thermally cracked Recombination of cracked aromatic clusters is possible, and in fact may become a precursor compound for production of more coke-forming materials. Growth of said clusters may be averted by combining with hydrogen radicals that is available in the reacting medium--commonly referred to as hydrogen capping or hydrogen stabilization. Most of the hydrogen atoms is produced by polymerization via dehydrogenation of aromatic compounds, but supplements of hydrogen radicals are also produced from thermal cracking of other hydrocarbons, e.g., naphthenes, olefins, paraffins, etc. Thus, a coker fresh feed with low hydrogen/carbon atom ratio produces more green coke and less hydrocarbons products. Current delayed coking operation has no control of increasing the hydrogen radicals except by charging the unit with lighter stocks of high hydrogen to carbon ratio or, in the case of U.S. Pat. Nos. 4,385,982 and 4,604,186 for example, a suitable hydrogen donor hydrocarbon is added into the process.
Impurities in the fresh feed residue such as metals, sulfur, and other heteroatoms are transferred into the products. Weakly bonded sulfur decompose and recombine with hydrogen radicals producing hydrogen sulfide gas. More thermally stable heteroatoms, that is, those atoms deeply imbedded in the mostly aromatic molecular structure of the fresh feed, go through heat treatment in the coke drum without decomposing and hence become part of the newly formed green coke and hydrocarbons products. Current delayed coking operation has limited control of reducing these impurities except by charging the unit with a coker stock of lesser heteroatoms content. The use of catalytic hydrogenation of coker stocks to remove some of these impurities are disclosed in U.S. Pat. Nos. 2,963,416 and 3,817,853.
Direct catalytic hydrogenation of the coker fresh feed to reduce the green coke yield and improve the hydrocarbons yield is disclosed in U.S. Pat. Nos. 3,684,688, 4,358,366 and 4,394,250. Hayashi disclosed in U.S. Pat. No. 4,312,742 a direct non-catalytic hydrogenation for improving green coke quality and for the removal of oxygen-, sulfur-, and nitrogen-containing impurities from the coker feedstock. The process of Hayashi consists of gradually heating the fresh feed, admixed with hydrogen gas, from approximately 300.degree. F. up to 752.degree. F. in furnace tubes within 30 to 180 minutes. The residue thus hydrotreated is transferred into a settling tank where the precipitated converted asphaltenes, other insolubles, and the unreacted hydrogen gas and inert gases, are separated from the clarified residue oil which is fed into a conventional coker heater and hence, into a conventional coke drum for delayed coking. It is emphasized that Hayashi has not disclosed and teached the embodiments involved in hydrogenation without gradually heating the coker fresh feed and without using a settling tank. Hayashi's use of the furnace heating tubes and a settling tank are essential to the removal of impurities from the fresh feed and consequently, to subsequent improvement of coke quality in the coke drum.
In aforementioned U.S. Pat. No. 3,817,853, Folkins disclosed the improvement in coke CTE and hydrocarbons yield by the catalytic hydrogenation of the coker fresh feed. Folkins further mentioned non-catalytic hydrogenation, but has not disclosed any embodiment whatsoever, as fittingly mentioned by Hayashi in aforementioned U.S. Pat. No. 4,312,742.